Change j2s to allow a configurable .j2s config file

Allow a property to be set when launching eclipse (headlessly) and a configuration in j2s to name that property.
The property contains a path/filename to use instead of ".j2s" for the j2s configuration.

This allows a headless gradle build of gradle to not change the .j2s file being used by Eclipse IDE development.

See https://github.com/BobHanson/java2script/pull/199